2,6-Dihydroxybenzoic acid and 2,4-dihydroxybenzoic acid, which are usually obtained in the form of a resorcylic acid mixture via the carboxylation of resorcin, are separated from each other and then employed as starting materials for producing drugs, agricultural chemicals and industrial chemicals.
As an example of a method for separating these compounds, Chemical Abstracts, vol. 69, 67115j (1968) and ibid, vol. 71, 91014n (1969) report a method which comprises adding an acid to a resorcylic acid mixture in an alkaline solution so as to adjust the pH value to 2.6 to 2.7, then recovering 2,4-dihydroxybenzoic acid precipitated earlier, further adjusting the pH value to 1.0 to 1.5 and then recovering 2,6-dihydroxybenzoic acid precipitated later.
In recent years, 2,6-dihydroxybenzoic acid is increasing in the range of its use more than 2,4-dihydroxybenzoic acid and moreover required to have high purity.
In this respect, as an example of a method for separating 2,6-dihydroxybenzoic acid from a resorcylic acid mixture, JP-A-62-440 discloses, in Example 1 thereof, the following method: An acid is added to an alkaline aqueous solution of a resorcylic acid mixture containing 50% or more of 2,6-dihydroxybenzoic acid so as to adjust the pH value to 4. After removing the insoluble matters thus formed, the pH value of the solution is further adjusted to 1 and then the 2,6-dihydroxybenzoic acid thus precipitated is recovered. The term "JP-A" as used herein means an "published unexamined Japanese patent application".
In Example 1 of British Patent No. 916,548, an resorcylic acid mixture in an alkaline aqueous solution is acidified with hydrochloric acid and the 2,6-dihydroxybenzoic acid precipitated earlier is recovered. The 2,6-dihydroxybenzoic acid thus recovered is next dissolved again in water at 75.degree. C. After removing insoluble matters, the solution is crystallized by cooling. Thus a purified product of 2,6-dihydroxybenzoic acid is obtained.
In Example 13 of the said British Patent, an alkaline aqueous solution of a resorcylic acid mixture is acidified and the precipitate thus formed is removed. Then seed crystals of 2,6-dihydroxybenzoic acid are added to the filtrate to deposit 2,6-dihydroxybenzoic acid, which is then separated.
In all the above-mentioned prior art methods, 2,6-dihydroxybenzoic acid is separated from 2,4-dihydroxybenzoic acid by controlling the pH value of an aqueous solution of a resorcylic acid mixture. In such methods, the resulting 2,6-dihydroxybenzoic acid is easy to be contaminated with 2,4-dihydroxybenzoic acid. Therefore, the 2,6-dihydroxybenzoic acid thus obtained has only a low purity and should be purified again.